Self-assembly patterning for fabricating thin-film devices

ABSTRACT

A method (200) for fabricating patterns on the surface of a layer of a device (100), the method comprising: providing at least one layer (130, 230); adding at least one alkali metal (235) comprising Cs and/or Rb; controlling the temperature (2300) of the at least one layer, thereby forming a plurality of self-assembled, regularly spaced, parallel lines of alkali compound embossings (1300, 1305) at the surface of the layer. The method further comprises forming cavities (236, 1300) by dissolving the alkali compound embossings. The method (200) is advantageous for nanopatterning of devices (100) without using templates and for the production of high efficiency optoelectronic thin-film devices (100).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Stage of InternationalApplication No. PCT/EP2017/051854, filed Jan. 27, 2017, whichapplication claims priority to International Application No.PCT/IB2016/050727, filed Feb. 11, 2016, the entire contents of which areincorporated by reference herein.

FIELD

The present invention relates to fabricating patterns at the surface ofdevices. It is applicable to solar cells, optoelectronic devices and/ordevices manufactured by deposition of thin-films and more particularlyto exploiting self-assembly properties of alkali halides to formfeatures at the surface of a substrative layer, for example the absorberlayer of optoelectronic devices comprising chalcogenide semiconductorsor ABC semiconductive compounds.

BACKGROUND

Fabricating patterns on the surface of devices may confer new orimproved properties and functions to said devices. Thin-films of alkalihalides may exhibit self-assembly properties that may be employed tocreate patterns at the surface of layers.

Photovoltaic devices are generally understood as photovoltaic cells orphotovoltaic modules. Photovoltaic modules ordinarily comprise arrays ofinterconnected photovoltaic cells. A thin-film photovoltaic oroptoelectronic device is ordinarily manufactured by depositing materiallayers onto a substrate. A thin-film photovoltaic device ordinarilycomprises a substrate coated by a layer stack comprising a conductivelayer stack, at least one absorber layer, optionally at least one bufferlayer, and at least one transparent conductive layer stack.

The present invention is also concerned with photovoltaic devicescomprising an absorber layer generally based on an ABC chalcogenidematerial, such as an ABC₂ chalcopyrite material, wherein A representselements in group 11 of the periodic table of chemical elements asdefined by the International Union of Pure and Applied Chemistryincluding Cu or Ag, B represents elements in group 13 of the periodictable including In, Ga, or Al, and C represents elements in group 16 ofthe periodic table including S, Se, or Te. An example of an ABC₂material is the Cu(In,Ga)Se₂ semiconductor also known as CIGS. Theinvention also concerns variations to the ordinary ternary ABCcompositions, such as copper-indium-selenide or copper-gallium-selenide,in the form of quaternary, pentanary, or multinary materials such ascompounds of copper-(indium, gallium)-(selenium, sulfur),copper-(indium, aluminium)-selenium, copper-(indium,aluminium)-(selenium, sulfur), copper-(zinc, tin)-selenium,copper-(zinc, tin)-(selenium, sulfur), (silver, copper)-(indium,gallium)-selenium, or (silver, copper)-(indium, gallium)-(selenium,sulfur).

The photovoltaic absorber layer of thin-film ABC or ABC₂ photovoltaicdevices can be manufactured using a variety of methods such as chemicalvapor deposition (CVD), physical vapor deposition (PVD), spraying,sintering, sputtering, printing, ion beam, or electroplating. The mostcommon method is based on vapor deposition or co-evaporation within avacuum chamber ordinarily using multiple evaporation sources.Historically derived from alkali material diffusion using soda limeglass substrates, the effect of adding alkali metals to enhance theefficiency of thin-film ABC₂ photovoltaic devices has been described inmuch prior art (RUDMANN, D. (2004) Effects of sodium on growth andproperties of Cu(In,Ga)Se₂ thin films and solar cells, Doctoraldissertation, Swiss Federal Institute of Technology. Retrieved 2014 Apr.30 from <URL:http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.pdf>).

Thin-films of alkali halides are known to form island structures(BOUTBOUL, T. et al. On the Surface Morphology of Thin Alkali HalidePhotocathode Films. Nuclear Instruments and Methods in Physics ResearchA 438 (1999) 409-414).

The present invention presents a method to form nanometric patterns, ornanopatterns at the surface of a device, for example an optoelectronicdevice's thin-film layer, preferably the device's absorber layer. Thepresent invention builds upon a previous filing (PCT/IB2015/056656claiming priority of 18 Sep. 2014) referred to as “previous filing” inthis description. The nanopatterns may comprise rows or arrays of alkalicrystals or compounds of alkali crystals formed by deposition of atleast one alkali material onto a thin-film layer. The nanopatterns mayalso comprise rows or arrays of cavities formed by selectivelydissolving said alkali crystals and underlying material that underwent areaction with said alkali crystals. Said nanopatterns are ordinarilyformed into a portion of the thickness of the thin-film layer onto whichthe alkali material is deposited. Said nanopatterns may also be formedinto at least one thin-film layer deposited after deposition of saidalkali material. The method may advantageously modify at least onethin-film layer's chemical composition, enlarge developed total surface,enlarge developed surface adequate for receiving doping elements, formpoint contacts with subsequently deposited thin-film layers, formpatterns in subsequently deposited thin-film layers, form patterns thatconfer specific properties to the device, form patterns that confer newfunctions to the device. The method may also enable tuning of the devicethrough control of pattern properties such as mean area, spatialdensity, spatial frequency, mean hole size, or area coverage, ofcavities, respectively alkali crystals.

In this document, we define alkali metal as being alkali metal inelemental form or precursor thereof.

The present invention exploits adding at least one alkali metal to alayer, for example to the absorber layer of a thin-film optoelectronicdevice. The invention especially exploits, under control of layertemperature, self-assembly properties of adding the at least one alkalimetal to a layer so as to form patterns. For example, adding at leastone alkali metal to a layer modifies at least the physical appearance ofthe surface of the layer and possibly also the chemical content of thelayer. Further treating of at least the surface of the absorber layerwill modify its physical appearance to reveal concave nanostructures ornanopatterns. Treating of absorber surface may for example be done witha bathing apparatus. The invention discloses independent control ofseparate alkali metals during adding to layers of the optoelectronicdevice, the treating of the absorber surface, and the resulting chemicaland physical modifications to at least one absorber layer of theoptoelectronic device. Effects of the invention on at least one of thedevice's thin-film layers include at least one of doping, passivation ofabsorber surface, interfaces, grain boundaries, and defects, elementalinterdiffusion, forming of point contacts, forming of nanoholes ofcontrolled dimensions and spatial distribution, modification of layerroughness, optical characteristics, and optoelectronic characteristicssuch as enhanced open circuit voltage and fill factor. The invention'sadding of at least one alkali metal and treating absorber surfaceenables manufacturing of a thinner optimal buffer layer. In some cases aperson skilled in the art may advantageously omit manufacturing thebuffer layer. This thinner optimal buffer layer results in reducedoptical losses, thereby contributing to increase an optoelectronicdevice's photovoltaic conversion efficiency.

SUMMARY

This invention presents a solution to the problem of, for example,precisely controlling and tuning the manufacturing of nanopatterns on alayer. The invention may especially be applicable to the manufacturingof high efficiency thin-film photovoltaic or optoelectronic devices thatcomprise an ABC₂ chalcopyrite absorber layer. The invention is alsoapplicable to flexible photovoltaic devices with said absorber layer. Itis also applicable to devices manufactured onto substrates, such aspolyimide, that do not comprise within the substrate alkali metals knownto augment photovoltaic conversion efficiency by diffusion into at leastthe absorber layer.

As an example for an application of manufacturing of nanopatterns on alayer, the invention presents photovoltaic (abbreviated PV) devicesthat, among alkali materials comprised in the absorber layer, comprisesan amount of Cs and/or Rb. The invention also describes somecharacteristics of said devices. The invention also presents a methodfor manufacturing said devices with the advantage of reduced opticallosses, reduced carrier recombinations in the absorber and at theinterfaces with the absorber, and therefore enhanced photovoltaicconversion efficiency. Although the method is applicable to productionon glass, metal, or various coated substrates, the method is especiallyadvantageous for the production of flexible photovoltaic devices basedon polymer substrates. Devices manufactured according to said methodhave higher photovoltaic efficiency, possibly lower temperaturecoefficient and possibly less unwanted material than equivalent devicesmanufactured using methods described in prior art.

A common problem in the field of thin-film (abbreviated TF) PV devicesrelates to doping of the photovoltaic absorber layer for increasedefficiency. When manufactured onto glass substrates or possibly ontosubstrates coated with materials comprising alkali metals, thesubstrate's alkali metals may diffuse into the absorber layer andincrease PV conversion efficiency. In the case of substrates, such aspolyimide, that do not comprise alkali metals, the alkali-dopingelements must be supplied via deposition techniques such as, forexample, physical vapor deposition. Alkali metals may for example besupplied as a so-called post deposition treatment. The alkali metalsdiffuse during the deposition process within and across various TFlayers and their interfaces.

Another problem in the field of TF PV devices concerned with dopingrelates to controlling doping of specific areas or specific zones alongthe thin-film's thickness of the semiconductive material.

A further problem in the field of TF PV devices concerned with dopingrelates to preparing the surface, for example relates to increasing theoverall surface, made available for doping.

Yet a further problem in the field of TF PV devices relates to damage tothe CIGS that may be caused by post-CIGS deposition treatments.

A problem in the field of TF PV devices is the occurrence of carrierrecombination which results in loss of PV conversion efficiency.

Another problem in the field of TF PV devices lies at the interfacesbetween the absorber layer, the optional buffer layer, and thefront-contact layer: semiconductive junction points that desirablyinterface as point contacts with high conductivity to the front-contactlayer must be well distributed across the layer's surface. It may beadvantageous to tune at least one of areal density, spatialdistribution, or spatial uniformity of said point contacts to thethin-films' semiconductive properties.

A further problem in the field of TF PV devices is that for some bufferlayer compositions, the thicker the buffer layer, the lower its opticaltransmittance and therefore the lower the PV device's conversionefficiency.

Yet a further problem in the field of TF PV devices is that some bufferlayer compositions, such as CdS, comprise the element cadmium, thequantity of which it is desirable to minimize.

Another problem in the field of TF PV device manufacturing is that theprocess for deposition of the buffer layer, such as chemical bathdeposition (CBD), may generate waste. In the case of CdS buffer layerdeposition the waste requires special treatment and it is thereforedesirable to minimize its amount.

Yet another problem in the field of flexible TF PV device manufacturingis that it is desirable to benefit from large process windows formaterial deposition, and more specifically in relation to thisinvention, the process window for the adding of alkali metals andsubsequent deposition of at least one buffer layer.

A problem in the field of TF device manufacturing is that it may bedesirable to manufacture thin-film layers with spatially uniformnanopatterns.

A problem in the field of TF device manufacturing is that it may bedesirable to manufacture a plurality of superimposed thin-film layerswith spatially corresponding nanopatterns.

A problem in the field of manufacturing nanopatterns in thin-filmdevices is that precise manufacturing of said nanopatterns may requirelabor-intensive and time-consuming processes to form features of saidnanopatterns.

Another problem in the field of manufacturing nanopatterns in thin-filmdevices is that precise manufacturing of said nanopatterns on top ofrough or polycrystalline surfaces may be desirable.

Finally, a problem in the field of TF PV devices is that of the color ofthe device and possibly also the uniformity of the color across thedevice's surface. This problem may be even more important in the contextof assemblies of PV devices, such as large PV modules, where multipledevices are placed next to each other and a desired match between thecolor of devices is desired. This may be for example to manufactureassemblies of PV devices of uniform color. It may also be to manufacturePV assemblies where different colors among PV devices are used to designpatterns, writings, or gradients.

Briefly, the invention thus pertains to a method of controlling andtuning the manufacturing of nanopatterns on a layer. The invention isfor example applicable to fabricating TF PV devices comprising at leastone ABC2 chalcopyrite absorber layer. The invention comprises adding atleast one alkali metal comprising Rb and/or Cs, thereby forming alkalicrystals that self-assemble and embed themselves into the surface of alayer or react with a small portion of the underlying material to giveit modified physical and chemical properties as that of the layer, suchas a PV device's absorber layer, to form embossings. The alkali crystalsmay further be selectively dissolved, thereby leaving cavities at thesurface of the layer in which initial embedding occurred. Embeddedalkali crystals may also be selectively dissolved after at least onesubsequent layer is deposited onto the layer where alkali crystals wereinitially embedded. The layers may also be subject to further treatmentwhich may also contribute to dissolving embedded alkali crystals.Resulting devices, or TF PV devices, such as those comprising at leastone ABC₂ chalcopyrite absorber layer, may be characterized as having onat least a region of their surface a plurality of cavities of nanoscopicscale. The cavities are arranged, through self-assembly, into parallellines of regularly distributed cavities, the cavities having a long axisthat is collinear with said line.

For the purposes of the present invention, the term “adding” or “added”refers to the process in which chemical elements, in the form ofindividual or compound chemical elements, namely Cs and/or Rb possiblywith other alkali metals and their so-called precursors, are beingprovided in the steps for fabricating the layer stack of a device, forexample an optoelectronic device, for any of:

-   -   forming patterns on the surface of at least one layer, said        patterns resulting from the forming and self-assembly of        aggregates comprising Cs and/or Rb possibly with other alkali        metals, or    -   forming a solid deposit where at least some of the provided        chemical elements will diffuse into at least one layer of said        layer stack, or    -   simultaneously providing chemical elements to other chemical        elements being deposited, thereby forming a layer that        incorporates at least some of the provided chemical elements and        the other elements, or    -   depositing chemical elements onto a layer or layer stack,        thereby contributing via diffusion at least some of the provided        chemical elements to said layer or layer stack.

In greater detail, the method for fabricating patterns on the surface ofa layer of a device comprises providing at least one layer; adding atleast one alkali metal comprising Cs and/or Rb; controlling thetemperature of the at least one layer, thereby forming a plurality ofembossings at the surface of the at least one layer, at least a portionof the plurality of embossings resulting from a self-assembly processcomprising: the forming of a plurality of alkali crystal compounds fromthe at least one alkali metal and the self-assembling and the embeddingof the alkali crystal compounds into the surface of the at least onelayer, thereby forming at least a first line of regularly spacedembossings that is adjacent and parallel to at least a second line ofregularly spaced embossings within at least one region of the at leastone layer.

In said method, at least one alkali metal comprises Cs and/or Rb. Thestep of adding of at least one alkali metal may also comprise adding atleast one element in group 16 of the periodic table including S, Se, andTe. Furthermore, the layer may comprise an ABC chalcogenide material,including ABC chalcogenide material quaternary, pentanary, or multinaryvariations, wherein A represents elements of group 11 of the periodictable of chemical elements as defined by the International Union of Pureand Applied Chemistry including Cu and Ag, B represents elements ingroup 13 of the periodic table including In, Ga, and Al, and Crepresents elements in group 16 of the periodic table including S, Se,and Te. The layer may for example comprise Cu(In,Ga)Se2. The step ofcontrolling the temperature may comprise controlling the temperature ina range from about 250° C. to about 380° C., preferably in a range fromabout 300° C. to about 370° C. The method may further comprise coatingat least a portion of the at least one layer with a functional layer.Furthermore, the method may comprise a step of forming at least onecavity by dissolving at least a portion of at least one of the alkalicrystal compounds, said step comprising, on at least a portion of alayer surface where at least a portion of at least one of the alkalicrystal compounds is exposed, at least one of the steps of: forming atleast one buffer layer; treating the layer surface by adding oxidationstate +1/+2 elements to the layer surface; aqueous wetting with asolution comprising water; aqueous wetting with a diluted aqueousammonia solution with a diluted ammonia molarity in the range from 0 to20 M, preferably in the range from about 1 M to 10 M, more preferably inthe range from about 2 M to 4 M. Additionally, the method may comprisefilling at least a portion of the at least one cavity by forming afiller layer onto at least one cavity. The layer may be deliveredbetween a delivery roll and a take-up roll of a roll-to-rollmanufacturing apparatus.

The inventive method can also comprise adding the alkali earth metalsBe, Mg, Ca, Sr and Ba in combination with Rb and Cs and possibly otheralkali metals, in the following proportions: 1/20000 to 10, preferably1/100 to 1/5. This may allow controlling the size and distribution ofthe self assembled crystals and resulting embossings.

The invention also pertains to a device obtainable by the describedmethod, comprising: at least one layer; a plurality of embossings at thesurface of said at least one layer, at least a portion of the pluralityof embossings resulting from a self-assembly process comprising: theforming of a plurality of alkali crystal compounds from at least onealkali metal and the self-assembling and the embedding of the alkalicrystal compounds into the surface of the at least one layer, therebyforming at least a first line of regularly spaced embossings that isadjacent and parallel to at least a second line of regularly spacedembossings within at least one region of the at least one layer. Also,the at least one alkali metal comprises Cs and/or Rb. The long axis ofat least one embossing in at least one line of regularly spacedembossings may be about collinear with said line. The at least one lineof regularly spaced embossings may be embossed along at least onecrystal striation of at least one region of the surface of the at leastone layer. At least one region may be comprised on a surface of acrystal at the surface of the at least one layer. At least one embossingmay be comprised in the at least one layer and at least one functionallayer. At least one embossing may have the shape of a cavity in at leastone layer. At least one embossing may comprise an electrical pointcontact between said substrative layer and at least one filler layer, atleast some material of which is comprised within said embossing. Atleast one layer may comprise a reacted layer. Furthermore, at least onelayer may comprise an ABC chalcogenide material, including ABCchalcogenide material quaternary, pentanary, or multinary variations,wherein A represents elements of group 11 of the periodic table ofchemical elements as defined by the International Union of Pure andApplied Chemistry including Cu and Ag, B represents elements in group 13of the periodic table including In, Ga, and Al, and C representselements in group 16 of the periodic table including S, Se, and Te. Theat least one layer may comprise Cu(In,Ga)Se2. The device may be athin-film optoelectronic device. The device may comprise a flexiblesubstrate.

ADVANTAGES

A main advantage of the invention is that it may enable, viaself-assembly, to control the production of uniform and orientedembossed nanopatterns on devices such as thin-film devices. Theself-positioning and uniform distribution of the embossed nanopatternsmay therefore be less labor-intensive than other methods that usetemplates, lithography, or laser drilling.

Another advantage of the invention is that it may enable, in a singlemanufacturing step with reduced and more efficient consumption ofchemical products, the fabrication of regular embossings in the absorberlayer surface of an optoelectronic device. The invention is alsoadvantageous for the single-step fabrication of regular embossings inmultiple layers that coat a substrative layer, such as an absorberlayer, where embossings have previously self-assembled to form regularnanopatterns. The invention contributes advantageous features of surfacenanostructuring, reduced surface damage, doping, buried and discretesemiconductive junction formation, and formation of point contacts. Theresulting devices, for example photovoltaic devices based on a CIGSabsorber layer, may also feature reduced or no cadmium content.

Advantages of the invention derive from a method of adding substantialamounts of alkali elements namely Cs and/or Rb to a layer, such as a PVdevice's absorber layer, and controlling the temperature to enable theself-assembly and embedding of regularly spaced and oriented alkalicrystals into the layer. A subsequent step may include selectivelydissolving the alkali crystals to form embossings of nanocavites andoptionally adding oxidation state +1 and/or +2 elements to the absorberlayer. High efficiency PV devices resulting from the method may beadvantageous, thanks to a thinner or an absence of buffer layer, overprior art devices where little or no alkali elements have been added.The method may also be advantageous over prior art devices where nonanocavities have been formed. An advantageous effect of the inventionis that the optimal thickness for an optional buffer layer coating saidabsorber layer may be thinner than the optimal buffer layer needed forprior art PV devices with comparable PV efficiency. The invention mayshorten the manufacturing process, reduce environmental impact ofmanufacturing and of the resulting device, and increase device PVconversion efficiency.

The invention's features may advantageously solve several problems inthe field of optoelectronics, TF PV devices manufacturing, and morespecifically manufacturing of the absorber and buffer layer of suchdevices. The listed advantages should not be considered as necessary foruse of the invention. For manufacturing of optoelectronic devices,preferably TF flexible PV devices, manufactured to the presentinvention, the advantages obtainable over devices and theirmanufacturing according to prior art include:

-   -   Higher PV conversion efficiency,    -   Improved absorber doping, especially towards the part of the        layer that is closest to the front-contact layer,    -   Control of doping spatial distribution    -   Absorber layer nanostructuring that improves dopant effect and        contributes to the formation of point contacts with the        front-contact layer that are regularly distributed and uniformly        sized,    -   Improved semiconductor junction that helps reduce carrier        recombinations,    -   Reduced semiconductor bulk defects that help reduce carrier        recombinations,    -   Tuning of areal density, spatial distribution, or spatial        uniformity of point contacts,    -   Forming of spatially uniform nanopatterns,    -   Forming of a plurality of thin-film layers with spatially        corresponding nanopatterns,    -   Fast and low-cost forming of nanopatterns in thin-film layers,    -   Thinner or omitted buffer layer,    -   Shorter buffer layer deposition time,    -   Enlarged buffer layer deposition process window,    -   Enlarged deposition process window for alkali metal doping        elements,    -   Increase in open-circuit voltage resulting from a decrease in        recombination-relevant surface,    -   Enlarged tuning window for (In, Ga) addition to the CIGS        absorber layer,    -   Improved copper to cadmium interface resulting in lower cadmium        content in device,    -   Facilitated growth of stoichiometric absorber material,    -   More environmentally-friendly manufacturing process and devices,    -   Lower manufacturing costs,    -   Adjustment of PV device color, spatial color uniformity, and/or        reflectance.

BRIEF DESCRIPTION OF FIGURES

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIG. 1A is a cross-section of an embodiment of a thin-filmoptoelectronic device.

FIGS. 1B-1J are partial device cross-sections detailing the formation ofembossings, FIGS. 1D, 1G and 1I being perspective views.

FIG. 2 presents steps in a method to manufacture a thin-film devicecomprising self-assembled patterns.

DETAILED DESCRIPTION

In simplified terms, the following description details a devicecomprising a layer that comprises a plurality of embossings formingnanopatterns. The device is preferably an optoelectronic device, forexample a TF PV device, and the layer is for example the device'sabsorber layer. The nanopatterns may comprise rows or arrays of alkalicrystals or alkali crystal compounds including Cs and/or Rb—in thefollowing the term alkali crystals will be used to designate anycompound comprising at least one crystal that comprises an alkalielement comprising Cs and/or Rb—formed by deposition of at least onealkali material onto a thin-film layer. The nanopatterns may alsocomprise rows or arrays of concave embossings, or cavities, formed byselectively dissolving said alkali crystals. The shape, location, andspatial distribution of the cavities, or nanocavities, correspond to thegeometry and topology of the alkali crystals prior to their dissolvingto form said cavities. In the following, metrics describing cavities mayordinarily be generalized to embossings. A convex embossing may beunderstood as an alkali crystal that is at least partly embedded into alayer used as a substrative layer, for example a PV device's absorberlayer. A cavity, or a concave embossing, may be understood as the shaperemaining in a substrative layer, and possibly subsequently depositedlayers, after subjecting an alkali crystal to selective dissolving. Thesize of an embossing may range from at least 2 to 800 nm, possiblylarger. The distance from a first embossing's centroid to a nearestsecond embossing's centroid, may range from 0 to 500 nm, possiblyfurther. Although the shape of the cavities is preferably rectangular,the shape more generally derives from cubic or aggregates of cubiccrystals. The shape may also be rectangular, tetragonal, pyramidal, andpossibly circular, or elliptic. The shape of cavities is a function ofstep temperatures and step durations used in the manufacturing method.The shape is also a function of the presence of other elements (forexample comprising Na, K and alkali earth elements) added simultaneouslyduring the adding of Cs and/or Rb that contribute to forming the alkalicrystals. The surface of the absorber layer may also be doped. Theforming of said crystals contributes to the doping. As an example formanufacturing an optoelectronic device, the description also details amethod to manufacture a TF PV device, parameters to grow the alkalicrystals that are embedded into the surface of the absorber layer of thedevice, how to selectively dissolve said crystals without damaging theabsorber layer, how to dope the surface of the absorber layer within theoverall manufacturing process, and how to control the nanopatterning ofthe absorber layer. In effect, by forming nanopatterns of cavities, themethod may uniformly increase the total surface available to dope thesurface of the absorber layer and at the same time reduce the volumewithin the absorber layer where detrimental carrier recombination mayoccur.

The invention is applicable to substrates that may or substrates thatmay not diffuse alkali elements into thin-film layers. For example themethod is applicable to alkali-nondiffusing substrates. An“alkali-nondiffusing substrate” is a component, ordinarily a sheet ofmaterial, that comprises no alkali or so little alkali that diffusion ofpotassium or other alkali elements into the subsequently describedlayers is considered too small to significantly alter the optoelectronicproperties of the device. Alkali-nondiffusing substrates also includesubstrates that comprise means to prevent diffusion of Cs and/or Rb intocoatings or layers supported by the substrate. An alkali-nondiffusingsubstrate may for example be a substrate that has been specially treatedor coated with a barrier layer to prevent diffusion of alkali elementsinto coatings or layers supported by the substrate. Specially treatedsubstrates or barrier-coated substrates ordinarily prevent the diffusionof a broad range of elements, including alkali metals, into coatings orlayers supported by the substrate.

For clarity, components in figures showing embodiments are not drawn atthe same scale.

FIG. 1A presents the cross-section of an embodiment of a TFoptoelectronic or a PV device 100 comprising a substrate 110 for a stackof material layers.

Substrate 110 may be rigid or flexible and be of a variety of materialsor coated materials such as glass, coated metal, polymer-coated metal,polymer, coated polymer such as metal-coated polymer, or flexible glass.A preferred flexible substrate material is polyimide as it is veryflexible, sustains temperatures required to manufacture high efficiencyoptoelectronic devices, requires less processing than metal substrates,and exhibits thermal expansion coefficients that are compatible withthose of material layers deposited upon it. Industrially availablepolyimide substrates are ordinarily available in thicknesses rangingfrom 7 μm to 150 μm. Substrate 110 may be an alkali-nondiffusingsubstrate. Polyimide substrates are ordinarily considered asalkali-nondiffusing.

At least one electrically conductive layer 120 coats substrate 110. Saidelectrically conductive layer, or stack of electrically conductivelayers, also known as the back-contact, may be of a variety ofelectrically conductive materials, preferably having a coefficient ofthermal expansion (CTE) that is close both to that of the said substrate110 onto which it is deposited and to that of other materials that areto be subsequently deposited upon it. Conductive layer 120 preferablyhas a high optical reflectance and is commonly made of Mo althoughseveral other TF materials such as metal chalcogenides, molybdenumchalcogenides, molybdenum selenides (such as MoSe2), Na-doped Mo,K-doped Mo, Cs-doped Mo, Rb-doped Mo, Na, K, Cs, and/or Rb (andcombinations thereof)-doped Mo, transition metal chalcogenides, dopedindium oxides, for example tin-doped indium oxide (ITO), non-dopedindium oxides, doped or non-doped zinc oxides, zirconium nitrides, tinoxides, titanium nitrides, Ti, W, Ta, Au, Ag, Cu, and Nb may also beused or included advantageously.

At least one absorber layer 130, which is also referred to herein as asubstrative layer 130, coats electrically conductive layer 120. Absorberlayer 130 is made of an ABC material, wherein A represents elements ingroup 11 of the periodic table of chemical elements as defined by theInternational Union of Pure and Applied Chemistry including Cu or Ag, Brepresents elements in group 13 of the periodic table including In, Ga,or Al, and C represents elements in group 16 of the periodic tableincluding S, Se, or Te. An example of an ABC2 material is theCu(In,Ga)Se2 semiconductor also known as CIGS.

Optionally, at least one semiconductive buffer layer 140 coats absorberlayer 130. Said buffer layer ordinarily has an energy bandgap higherthan 1.5 eV and is for example made of materials such as cadmiumsulfides, CdS, Cd(S,OH), CdZnS, indium sulfides, zinc sulfides, galliumselenides, indium selenides, compounds of (indium, gallium)-sulfur,compounds of (indium, gallium)-selenium, tin oxides, zinc oxides,Zn(Mg,O)S, Zn(O,S) material, or variations thereof.

At least one transparent conductive front-contact layer 150 coats bufferlayer 140. Said transparent conductive layer, also known as thefront-contact, ordinarily comprises a transparent conductive oxide (TCO)layer, for example made of doped or non-doped variations of materialssuch as indium oxides, tin oxides, or zinc oxides, indium tin oxides,fluorine-doped tin oxides, hydrogen-doped indium oxides, doped indiumoxides. Optionally, the place of buffer layer 140 may be replaced byother types of transparent layers, for example a transparent conductivefront-contact layer 150.

Contributing to this invention, for example when applied to anoptoelectronic device, preferably a PV device, the amount of Cs and/orRb comprised in the interval of layers 370 from electrically conductiveback-contact layer 120, exclusive, to transparent conductivefront-contact layer 150, inclusive, the comprised amounts resulting fromadding the alkali metals are, for Rb and/or Cs, in the range of 200 to10000, preferably 500 to 2500, more preferably 1000-2250 or 1500-2000atoms per million atoms (ppm) and, where another alkali metal ispresent, the other alkali metal(s) is/are in the range of 5 to 10000 ppmand at most 3/2, preferably at most 1/2, and at least 1/2000 of thecomprised amount of Rb and/or Cs. A TF PV device demonstrating superiorPV conversion efficiency preferably has an amount of Cs and/or Rbcomprised in said interval of layers 370 in the range between 500 and2000 Cs and/or Rb atoms per million atoms. For a device comprising atleast two alkali metals, one of which is Cs and/or Rb, the amount ofsaid at least one alkali metal other than Cs and/or Rb may be in therange of 5 to 5000 ppm. the amount of at least one alkali metal otherthan Cs and/or Rb is at most 3/2, preferably 1/2 and at least 1/2000 ofthe comprised amount of Cs and/or Rb.

Optionally, front-contact metallized grid patterns 160 may cover part ofthe transparent conductive front-contact layer 150 to advantageouslyaugment front-contact conductivity. Also optionally, said TF PV devicemay be coated with at least one anti-reflective coating such as a thinmaterial layer or an encapsulating film.

FIG. 1B presents a detail of a partial cross-section of an embodiment ofa device 100 with a layer 130 coated by a plurality of alkali crystals1320 including Cs and/or Rb based crystals. The partial cross-section ispreferably a partial cross-section of a TF optoelectronic device, forexample the TF PV device 100 of FIG. 1A. FIG. 1B shows part of a layer130 acting as a substrative layer 130 for alkali crystals 1320. In thiscontext, the alkali crystals 1320 form a plurality of embossings, inthis context convex embossings 1305, onto at least the substrative layer130. The substrative layer 130 is preferably a thin-film layer, forexample the absorber layer 130 of FIG. 1A. The substrative layer 130 maycomprise a plurality of substrative layer crystals, each substrativecrystal comprising regions of substrative layer crystal 135 where aplurality of said embossings may form. The surface of the substrativelayer therefore comprises a plurality of regions of substrative layercrystals 135 that expose at least parts of faces of the substrativelayer's crystals. For example, if substrative layer 130 is a CIGSabsorber layer, said substrative crystals are chalcopyrite grains, orcrystallites. The surface of substrative layer 130 is coated by aplurality of alkali crystals 1320. The alkali crystals are preferablynon-connected single crystals. The alkali crystals may also comprisenon-connected assemblies of alkali crystals, for example assemblies ofcrystals may comprise twinned crystals. Said alkali crystals preferablyhave a shape resembling a rectangular parallelepiped. The alkalicrystals may at least be partly embedded into the surface of theunderlying substrative layer 130. Preferably, a plurality of alkalicrystals may be aligned and form a line 1340 of embossings 1300. Saidline of embossings may be oriented along a direction of line ofembossings 1345. In FIG. 1B the line of embossings comprises the alkalicrystals and therefore forms a line of convex embossings. The surface ofthe layer 130 may optionally comprise a portion of reacted layer 1310.For example, in the case of a CIGS absorber layer 130, the reacted layer1310 may comprise Se and may also comprise elements diffusing from thesubstrative layer. The reacted layer 1310 therefore comprises acombination, or a mixture, of alkali salts, Cu, In, Ga, and/or Se.

FIG. 1C presents the partial cross-section of an embodiment of a device100 of FIG. 1B after selective dissolving of at least one alkalicrystal, ordinarily a plurality of alkali crystals. The dissolvingresults in forming at least one cavity of an embossing 1300, ordinarilya plurality of cavities of embossings 1300. The cavities may beunderstood as the concave embossings 1300 that are counterpart to theconvex embossings 1305 of FIG. 1B.

The embossings, whether they are convex embossings 1305 comprisingalkali crystals 1320 of FIG. 1B or concave embossings 1300 of FIG. 1C,may be immersed at various depths into the layer 130: they may bepositioned against the surface of the layer 130, partly immersed intothe reacted layer 1310, partly immersed into the layer 130 and partlyimmersed below the reacted layer 1310, immersed to be flush with thesurface of the layer, immersed to be flush with the surface of thereacted layer, or even immersed below the overall surface of the layeror the reacted layer within the corresponding region of the substrativelayer crystal 135. A given line 1340 of embossings 1300, 1305 maycomprise embossings that are immersed over a plurality of depths intothe layer 130.

Also, a portion of the surface of absorber or substrative layer 130 maycomprise at least one oxidation state +1 element and/or at least oneoxidation state +2 element (see description for FIG. 2). Said oxidationstate elements may for example be present at the location of at leastone embossing, more preferably the location of at least one concaveembossing. On the cross-section of the TF device, the surface of theabsorber or substrative layer 130 may be defined as the region in whichthe embossings 1300, 1305 extend into the layer and additionally up toabout 80 nm, preferably 50 nm, further into the layer 130. Consequently,layer 130 may mostly be a p-type absorber. At its surface, for exampleat the location of at least one embossing, in contact with at least oneof buffer layer 140 or front-contact layer 150, it may comprise n-typeabsorber material. Layer 130 may therefore comprise at least onehomojunction, preferably a plurality of homojunctions. Saidhomojunctions may be located at the embossings, preferably concaveembossings, preferably within the absorber layer material surroundingsaid embossings comprised in a radius of about 100 nm. An example ofhomojunction for an ABC2 absorber layer is a p-type CIGS absorber layercomprising n-type CIGS at the surface. Furthermore, the surface of theabsorber layer, for example at the location of at least one concaveembossing, may also comprise at least one heterojunction, preferably aplurality of heterojunctions. Said heterojunction may for examplecomprise a p-type CIGS absorber layer combined with n-type material suchas CdS, Cd(S,OH), Cd(O,H), CdZnS, KInSe2, CsInSe2, RbInSe2, indiumsulfides, zinc sulfides, gallium selenides, indium selenides, compoundsof (indium, gallium)-sulfur, compounds of (indium, gallium)-selenium,tin oxides, zinc oxides, Zn(Mg,O)S, Zn(O,S), alkali selenides, alkalisulfides, or variations thereof.

FIG. 1D is a perspective view of a region of a substrative layer crystal135 comprising a plurality of concave embossings 1300, or cavities. Theregion of a substrative layer crystal 135 comprises at least one line1340 of regularly spaced embossings 1300. Said region 135 may alsocomprise at least a first line 1340 of regularly spaced embossings 1300that is parallel to at least a second line 1340 of regularly spacedembossings 1300. Said region 135 may also comprise a set or mesh 1440comprising a plurality of parallel lines 1340, each line comprisingregularly spaced embossings 1300. In said set 1440, the spacing betweenembossings in a first line 1340 may be different from the spacing ofembossings in a second line 1340. Said region 135 may therefore resultin a mesh of substrative layer 130 into which embossings 1300, 1301 haveformed. More generally, a device may comprise a plurality of regionswith a plurality of embossing patterns arranged in a plurality ofdirections. A device may also comprise regions with embossing aligned inpatterns and regions where embossings appear randomly positioned.

Crystals of the substrative layer 130 may comprise crystal striations1350 that may act as guides for forming lines 1340 of embossings 1300,1305. Said crystal striations 1350 are ordinarily naturally occurringformations resulting from the growth of the crystals that form thesubstrative layer 130. The crystal striations may therefore contributeto the self-assembly of alkali crystals 1320 and enable self-positioningand self-alignment of the alkali crystals and subsequent embossingswithout having to use template-based or lithographic methods. Theorientation 1345 of at least one line 1340 in said region 135 may beparallel to that of at least one crystal striation 1350 of at least oneregion 135 of the surface of the at least one layer 130. A line 1340 ofregularly spaced embossings may adopt a variety of positions withrespect to a crystal striation 1350: said line 1340 may for exampleoverlap and be collinear with at least one crystal striation 1350; saidline may comprise at least one embossing, the long side 1302 of which isflush against at least one crystal striation; said line may be comprisedbetween at least two said crystal striations and parallel to at leastone crystal striation. The long side 1302 of at least one embossing1300, 1305 has an orientation 1341 that is about parallel to the nearestcrystal striation 1350. The orientation 1341 of a long side 1302 of atleast one embossing 1300, 1305 may therefore be parallel to theorientation 1345 of at least a portion of a line 1340 of embossings. Ina given line 1340 on the surface of the substrative layer 130, the longside 1302 of a first embossing 1300, 1305 may be parallel to the longside of at least a second adjacent but non-touching embossing 1300,1305. More generally, embossings in a given line 1340, or a given set1440, may have the same orientation 1341. FIG. 1D also presents at leastone optional deeper embossing 1301 that is formed deeper into thesubstrative layer 130, for example with at least a portion that is belowthe reacted layer 1310 of the substrative layer, than other embossings1300 on the same line 1340. Embossings manufactured on a CIGSsubstrative layer with the objective of forming uniformly distributedpoint contacts ordinarily have a length along direction 1341 rangingfrom about 5 nm to about 40 nm. A person skilled in the art ofmanufacturing devices comprising a CIGS absorber layer with a carrierlifetime of 1-100 ns will preferably choose to manufacture a highphotovoltaic conversion efficiency device where embossings have a lengthcomprised between about 5 nm and 20 nm, preferably about 10 nm with adistance between the centroid of a first embossing and the centroid of anearest second embossing of about 35 nm to 65 nm, preferably about 50nm.

FIG. 1E presents the device of cross-section of FIG. 1B where afunctional layer 1380 has been added to cover at least part of thesurface of substrative layer 130. The coating may therefore also coatpart of the reacted layer 1310 of substrative layer 130. The coating mayalso at least partly cover at least one, ordinarily a plurality, ofembossings 1305 comprising alkali crystals 1320. The coating maypreferably not entirely cover embossings 1305 so as to facilitatesubsequent dissolving of alkali crystals 1320. The coating maypreferably comprise material to constitute at least one functionallayer. Types of functional layers may comprise so-called passivationlayer, buffer layers, or conductive layers. Materials for a passivationlayer may comprise oxides such as aluminum oxides, silicon oxides,nitrides such as silicon nitrides, or amorphous silicon. Materials for abuffer layer may comprise those listed for buffer layer 140. Materialsfor a conductive layer may comprise those listed for the transparentconductive front-contact layer 150, may comprise metals such asmaterials comprising silver, gold, platinum, copper, indium, aluminum,nickel, and/or may materials listed for electrically conductive layer120.

FIG. 1F presents the device of cross-section of FIG. 1E after removal ofat least one alkali crystal 1320, preferably a plurality of alkalicrystals. Removal of at least one alkali crystal ordinarily also resultsin removing part of the material of the functional layer 1380 thatcovers the alkali crystal. The removal of at least one alkali crystalresults in forming at least one concave embossing 1300. The resultingdevice therefore comprises at least one region comprising at least oneembossing into at least one functional layer 1380 that extends into atleast the substrative layer 130. When viewed from a direction that isperpendicular to the surface of the region 135 where the embossing 1300is formed, the contour of the embossing in the functional layer isordinarily about the same as that in the underlying substrative layer.However, a device may also comprise at least one embossing that does nothave the same contour in the functional layer as in the underlyingsubstrative layer, thereby forming at least one partly closed embossing1330. This may happen, for example, if upon forming the concaveembossing 1300 the functional layer is sufficiently thick over thealkali crystal to remain attached to the rest of the substrative layer130, functional layer 1380 into which the embossing is formed.

FIG. 1G is a perspective view of a region of the device presented inFIG. 1F. The device in this figure corresponds to that of FIG. 1D butrepresented in this figure with a functional layer 1380 that coats thesubstrative layer 130, its optional reacted layer 1310, and is similarlypatterned with embossings 1300, 1301. The device therefore at leastcomprises features presented in FIG. 1D but here with the additionalfunctional layer. The device comprises a plurality of lines 1340 ofregularly spaced embossings 1300, 1301. The device may also compriseoptional deeper embossings 1301. A region of the device may thereforecomprise at least one set 1440 comprising a plurality of parallel lines1340 of the embossings described in FIG. 1F.

FIG. 1H presents a device 100 comparable to that of cross-section ofFIG. 1F that additionally comprises at least one filler layer 1360 thatcoats at least one region of the device. Filler layer 1360 may forexample have a composition or comprise materials that are similar tothose of the buffer layer 140 or of the front-contact layer 150.Although FIG. 1H is illustrated with a functional layer 1380, saidfunctional layer may be considered as optional. Material of the fillerlayer 1390 may at least partly fill at least one embossing 1300, 1301,preferably completely fill at least one embossing, more preferably filla plurality of embossings. Embossings 1300, 1301 may comprise at leastone electrically conductive pathway from substrative layer 130 to fillerlayer 1360.

FIG. 1I presents a device 100 comparable to that of FIG. 1G. The devicein this figure comprises embossings 1300′, 1301′ that are filled withfiller material. For an optoelectronic device, the filler material mayfor example be material acting as buffer material. The filler material,which may fill at least one embossing in at least one of layers 130,reacted layer 1310, functional layer 1380, is preferably flush with thesurface of at least one of layers 130, reacted layer 1310, or functionallayer 1380.

FIG. 1J presents a cross section of the device 100 of FIG. 1I aftercoating with a front-contact layer 150. At least one conductive layer120 and a substrate 110 are also represented. Filled embossings 1300′,1301′ therefore provide point contacts as electrical pathways from theabsorber or substrative layer 130 to the front-contact layer 150.

FIG. 2 presents a method 200 comprising material deposition steps tomanufacture said device 100, for example a TF optoelectronic or PVdevice, comprising a plurality of embossings forming nanopatterns. Themethod is considered to be especially appropriate for substratesconsidered alkali-nondiffusing or that may comprise at least one barrierlayer that prevents the diffusion of alkali metals from the substrateinto subsequently deposited coatings. The method as described isapplicable to production on glass, metal, or various coated substrates,and is especially advantageous for polymer substrate materials such aspolyimide.

An exemplary sequence of material layer deposition and treatmentfollows. The purpose of this description is to clarify the contextwithin which forming of a plurality of self-assembled embossings, themain subject of this invention, occurs. The description also focuses onforming concave embossings, also called cavities. Between any of thesubsequent manufacturing steps, a person skilled in the art will knowhow to, as an option, temporarily store the unfinished product, forexample within a vacuum or near vacuum container or even within acontainer characterized by a controlled atmospheric environment orpossibly a container comprising at least one inert gas. Exposure to airand/or to humidity is known to have an effect on the chemicalcomposition of the material layers deposited during the materialdeposition steps. A person skilled in the art will advantageously usesteps of the method within a manufacturing sequence where exposure toair and/or humidity is kept to a minimum between steps. A person skilledin the art may also use steps of the method so that the intermediateproduct is only exposed to a controlled environment between steps, suchas vacuum, near vacuum, low humidity atmosphere, or at least one inertgas. In case of exposure to air between steps that is different from airat standard ambient temperature and pressure (SATP) and 40% relativehumidity (RH), a person skilled in the art will adapt the limits forsaid cumulated minutes in air according to changes in aforementionedenvironmental parameters. In the rest of this document, the word minutesis abbreviated as min.

The method starts at step 210 by providing a substrate. Said substrateis an alkali-nondiffusing substrate.

Following step 210 and prior to the step of forming cavities 236, addingof at least one alkali metal 235 occurs as at least one event duringand/or between any of the steps comprised in the interval from the step210 of providing substrate (excluding the step 210 itself), to the stepof controlling temperature 2300 (excluding the step 2300 itself). Saidat least one said alkali metal comprises Cs and/or Rb. The fact that theadding may occur during or between said interval of steps is representedby dashed arrows emanating from block 235 in FIG. 2. Each of said alkalimetals may be added simultaneously with any of the other of said alkalimetals and/or during separate adding events. Adding of each of saidalkali metals may comprise any or a combination of adding a layer orprecursor layer of at least one of the alkali metals, co-adding at leastone of the alkali metals with the forming of any of the method'smaterial layers, or diffusing at least one of the alkali metals from atleast one layer into at least one other material layer. Preferably,adding of at least one different alkali metal is done in the presence ofat least one said C element. More preferably, adding of at least onedifferent alkali metal may be simultaneous to adding at least one said Celement. C elements comprise elements in group 16 of the periodic tableincluding S, Se, or Te. More preferably, adding of Cs and/or Rb, forexample by adding via a so-called Cs and/or Rb-comprising precursor suchas Cs and/or RbF, Cs and/or RbCl, Cs and/or RbBr, Cs and/or RbI, Csand/or Rb2S, Cs and/or Rb2Se, is done in the presence of at least onesaid C element.

At step 220, forming at least one back-contact layer comprisesdepositing at least one electrically conductive layer. Forming of theback-contact layer may be done using a process such as sputtering,spraying, sintering, electrodeposition, CVD, PVD, electron beamevaporation, or spraying of the materials listed in the description ofsaid electrically conductive layer 120.

The providing layer step 230 may for example comprise forming at leastone substrative layer. For example, for forming an optoelectronicdevice, the providing layer step may comprise coating said electricallyconductive layer with at least one absorber or substrative layer 130.Said substrative layer may for example be an ABC substrative layer 130.The materials used correspond to those in the description provided foran ABC substrative layer 130. Said substrative or absorber layer may bedeposited using a variety of techniques such as sputtering, spraying,sintering, CVD, electrodeposition, printing, or as a preferred techniquefor an ABC material, physical vapor deposition. Substrate temperaturesduring absorber layer deposition are ordinarily comprised between 100°C. and 650° C. The range of temperatures and temperature change profilesdepend on several parameters including at least the substrate's materialproperties, the supply rates of the materials that compose the ABCmaterial, and the type of coating process. For example, for a vapordeposition process, substrate temperatures during forming of theabsorber layer will ordinarily be below 600° C., and if using substratesrequiring lower temperatures, such as a polyimide substrate, preferablybelow 500° C., and more preferably in the range from 100° C. to 500° C.For a co-evaporation vapor deposition process, substrate temperaturesduring forming of the absorber layer will ordinarily be in the rangefrom 100° C. to 500° C. Said substrate temperatures may beadvantageously used with a polyimide substrate.

The substrative layer may also be coated with other layers such as atleast one layer that may function as a buffer layer, an indium selenidelayer, and/or a gallium selenide layer.

For a deposition process such as physical vapor deposition, for exampleif providing layer step 230 is done using a physical vapor depositionprocess, adding of Cs and/or Rb as part of adding at least one alkalimetal 235 may be done during and/or in continuation of the physicalvapor deposition process by supplying Cs and/or Rb fluoride, Cs and/orRbF. This may, for example, be advantageous when manufacturing with aco-evaporation physical vapor deposition system. Adding the alkali metalCs and/or Rb will preferably be done in the presence of a flux ofelement Se supplied at a rate in the range of 5 to 100 Å/s, preferablyat a rate in the range of 20 to 50 Å/s.

At step 2300, controlling temperature during or after the step of addingat least one alkali metal 235 comprising Cs and/or Rb enables theforming and self-assembly of embossings onto substrative layer 130. Aperson skilled in the art may control manufacturing parameters such asi) the evolution of temperatures during and after adding at least onealkali metal including Cs and/or Rb, ii) the order in which a pluralityof alkali metals via a plurality of steps of adding at least one alkalimetals is done, and iii) the amount of the at least one alkali metalsupplied during the adding at least one alkali metal. Control ofmanufacturing parameters enables control of at least the average length,area, aggregation of embossings, and over the layer's surface, thedensity, the separation, the covered area, the lining up, and thepattern forming of embossings.

Substrate temperature ranges for the step of controlling temperature orfor said adding of at least one alkali metal comprising Rb and/or Cs arefrom 100° C. to 700° C., preferably from 200° C. to 450° C., morepreferably from 300° C. to 400° C. For better results, the step ofcontrolling temperature comprises controlling the temperature between arange from about 250° C. to about 380° C., preferably between a rangefrom about 300° C. to about 370° C. A person skilled in the art willselect appropriate temperatures for said adding of at least one alkalimetal so that they are compatible with the materials deposited, TFproperties, and substrate. For example, adding of at least one alkalimetal may be with a physical vapor deposition process where an alkalimetal precursor comprising Cs and/or Rb for example in the form of CsFand/or RbF, is supplied at a rate equivalent to an effective layerdeposition of about 1 nm/min to 2 nm/min for a duration of 20 minutes.The skilled person will also know that adding of at least one alkalimetal comprising Cs and/or Rb may take place with adding of one or moreof said at least one alkali metal at substrate temperatures ordinarilylower than 700° C. and possibly much lower than 350° C., such as atambient temperatures of about 25° C. and below. The substrate may thenbe heated afterwards, thereby facilitating diffusing of said alkalimetals to the TF layers of the optoelectronic device, possibly incombination with depositing at least one C element. Adding at least onealkali metal 235 is preferably done in the presence of Se.

The amount of Cs and/or Rb added by adding at least one alkali metal 235is such that following forming of front-contact layer 150 at later step250, said amount comprised in the interval of layers 370 fromback-contact layer 120, exclusive, to front-contact layer 150,inclusive, is in the range between 200 and 10000 Cs and/or Rb atoms permillion atoms and the amount of other alkali metals is in the range of 5to 10000 ppm and at most 3/2 preferably 1/2 and at least 1/2000 of thecomprised amount of Cs and/or RbCs and/or Rb. A TF PV device that has asuperior PV conversion efficiency preferably has an amount comprised insaid interval of layers 370 from about 500 to 2000 Cs and/or Rb atomsper million atoms.

A person skilled in the art may advantageously use the step ofcontrolling temperature 2300 in accordance with the step of adding atleast one alkali metal 235 comprising Cs and/or Rb. For example, one mayform a high density of embossings on a layer surface by, in the step ofadding at least one alkali metal 235, adding Na before adding Cs and orRb. The resulting embossings ordinarily have a length that is comprisedbetween about 10 nm and 100 nm, preferably about 20 nm. However, thegreater the amount of Cs and/or Rb, the more alkali crystals are formed.The more alkali crystals are formed, the greater the probability offorming crystal aggregates and/or forming embossings that contact eachother on the layer surface, thereby forming larger aggregate embossings.A person skilled in the art may therefore control the average area orsize of embossings as a function of the amount of Cs and/or Rb added inthe step of adding at least one alkali metal 235.

As a guiding principle for the skilled person applying the method tomanufacture photovoltaic devices with a higher photovoltaic efficiency,an objective for raising efficiency is to try raise open circuit voltageVOC by trying to manufacture concave embossings that have the smallestpossible size or length. However, another objective is to maximize thedevice's fill factor (FF) and therefore determine the optimal distancebetween the centroid of a first embossing and the centroid of a nearestsecond embossing. Optimization of this distance is a function of theelectrical resistance between the absorber layer and at least onesubsequently deposited layer, for example a buffer layer, possibly alsoa front-contact layer.

At step 2350, that of forming a functional layer, a functional materialmay optionally be deposited to form a functional layer 1380. Thematerial may for example confer a buffer function between thesubstrative or absorber layer and a subsequently deposited front-contactlayer. If the objective is to subsequently form concave embossings 1300,the functional layer is preferably sufficiently thin so as to enable theoptional subsequent step of forming cavities 236. Therefore, at least aportion of the alkali crystals 1320 that forms at least one embossing1305 should preferably remain exposed, that is, preferably not coated bymaterial of functional layer 1380 so as to enable processing atsubsequent step 236. Materials that may be used for forming functionallayer 1380 comprise those listed in the description for FIG. 1E. Methodsthat may be used for forming the functional layer comprise CVD, PVD,sputtering, sintering, electrodeposition, printing, atomic layerdeposition, or as a well known technique at atmospheric pressure, CBD.

At step 236, that of forming cavities, at least the surface ofsubstrative layer or absorber layer 130 is subject to at least one ofthe steps of aqueous wetting 237, treating layer surface 238, or formingbuffer layer 240. At step 236 at least one, preferably a plurality, ofalkali crystals embedded within the surface of the substrative layer isdissolved. Part of the substrative layer material directly underlying atleast one of said alkali crystal comprising Cs and/or Rb may also bedissolved, said part will ordinarily not extend deeper into thesubstrative than one height unit corresponding to the depth of thecrystal embedded into the substrative layer. Also, part of the surfaceof the substrative layer directly in contact with said alkali crystalsmay be dissolved. Furthermore, part of the surface of the substrativelayer may also be dissolved.

At optional step 237, represented as a dashed box because the step maybe considered optional, aqueous wetting comprises wetting at least thesurface of substrative layer 130 with at least one aqueous wettingmaterial or solution. Aqueous wetting will preferably be done in a bath.Aqueous wetting may also be done using spraying. Aqueous wetting ispreferably with a diluted aqueous ammonia solution. Exposure to air isknown to have an effect on the chemical composition of the substrativelayer. A person skilled in the art will try to minimize the duration ofexposure of the device to air. Although the duration of exposure may beof several days, aqueous wetting preferably occurs after the substratehas spent a duration of at most 10, more preferably less than 5,cumulated minutes in air at SATP and 40% RH after completion of theproviding layer step 230. Described briefly, composition of thepreferred diluted aqueous ammonia solution bath is an aqueous solutionwith a molarity in the range from 3 M to 5 M. Said diluted aqueousammonia solution comprises a mixture of water and commercially availableammonia aqueous solution. Parameters relevant to the aqueous wettingstep comprising a bath comprise a cumulated duration of about 10 min,preferably, about 2 min, a bath temperature of about 25° C. and amolarity of about 3 mol/L (also written M). Ranges, preferred ranges,and most preferred ranges for said parameters are presented in Table 1below. Most preferred ranges are at least applicable using a bath. Aperson skilled in the art will readily adapt parameters to other typesof wetting apparatuses.

Wetting steps may take place in a chemical bath deposition (CBD)apparatus and/or a spraying apparatus. A chemical bath deposition systemensures continuous flow and mixing of the bathing solution over at leastsubstrative layer 130.

At the steps for treating layer surface 238 and/or forming buffer layer240, so-called “oxidation state +1” and/or so-called “oxidation state+2” elements, thereafter abbreviated +1/+2 elements, are added to thesubstrative layer 130 and especially to the surface of the substrativelayer. For a substrative layer that comprises an absorber layer, addingof oxidation state +1/+2 elements to the surface of the substrativelayer transforms at least a portion of the surface from a p-typeabsorber surface into an n-type absorber surface, thereby forming aburied junction at at least a portion of the absorber layer. Said buriedjunction may be a homojunction. Said buried junction is preferably a p-njunction. Resulting presence of oxidation state +1/+2 elements into theabsorber layer may then, for example, be the result of physisorption,chemisorption, or diffusion. Said oxidation state +1/+2 elementscomprise at least one element of at least one of group 2, group 3,lanthanide series, actinide series, or transition metals from theperiodic table of elements. A commonly used oxidation state +1/+2element is cadmium. Although this description focuses mostly onsolutions comprising cadmium, a person skilled in the art may adapt theinvention to use steps that comprise other or additional oxidation state+1/+2 elements. For example, a person skilled in the art may want toreduce or eliminate the amount of cadmium comprised in the resulting PVdevice by replacing part or all of the cadmium used in the method withat least one other oxidation state +1/+2 element.

At step 238, represented as a dashed box because the step may beconsidered optional, at least substrative layer 130 is subject to atleast one treating layer surface 238. The step of treating layer surface238 is especially useful if the step of forming buffer layer 240 isomitted. The duration of exposure to air between aqueous wetting 237 andtreating layer surface 238 is preferably at most 2, preferably less than0.5, cumulated minutes in air at aforementioned SATP and RH of step 237.Described briefly, relevant parameters for the step of treating thelayer surface comprise treating for a cumulated duration of about 22minutes at least substrative layer 130 into a solution with temperatureof about 70° C. comprising, per liter of water, an amount of about 60 mLof cadmium (Cd) solution and an amount of about 140 mL of ammonia (NH₃)solution. Ranges for said parameters are presented in Table 1. Saidwater is preferably distilled water or deionized water, more preferablyultra-pure water with a resistivity of about 18 MΩ·cm. Said cadmiumsolution comprises a Cd salt solution with a molarity most preferably inthe range from about 0.026 to 0.03 mol/L. Concentration of said ammoniasolution is more preferably in the range from 14.3 to 14.7 M. Ranges,preferred ranges, and most preferred ranges for said parameters arepresented in Table 1. Most preferred ranges are at least applicableusing a bathing apparatus.

TABLE 1 Ranges for steps 237 to 246. Range Pref, range More pref, rangefrom-to from-to from-to Step 237: Aqueous wetting Duration [min]0.05-7200  0.1-30  1-5 Temperature [° C.] 2-95 10-50 23-27 Dilutedammonia molarity [mol/L] 0-20  1-10 2-4 Step 238: Treating layer surfaceDuration [min] 0.05-7200  0.1-30  20-24 Temperature [° C.] 2-95 10-8565-75 Amount of Cd solution per liter [mL]  10-1000  20-100 55-65 Amountof NH₃ solution per liter [mL]  0-990  20-400 130-150 Cd molarity in Cdsolution [mol/L] 0.00001-10     0.01-0.1  0.026-0.03  Ammonia solutionmolarity [mol/L] 1-30 10-20 14.3-14.7 Step 240: Forming buffer layerDuration [min] 0.05-7200  0.1-30  14-18 Temperature [° C.] 2-95 10-8565-75 Cd salt solution molarity [mol/L] 0.00001-10     0.01-0.1 0.026-0.030 Ammonia solution molarity [mol/L] 1-30 10-20 14.3-14.7Thiourea aq. sol. molarity [mol/L] .01-10  0.2-0.6 0.35-0.4  Step 244:Annealing Cadmium sulfide, temperature [° C.] 100-300  150-200 175-185Cadmium sulfide, duration [min] 1-30 1-5 1.8-2.2 Zinc oxi-sulfide,temperature [° C.] 100-300  150-250 190-210 Zinc oxi-sulfide, duration[min] 1-30  5-15  9-11 Step 246: Degassing Degassing temperature [° C.]10-150 20-60 28-32 Degassing duration [min]  0.1-3000 1440-28801800-2400

At optional step 240, represented as a dashed box because the step maybe considered optional, forming the buffer layer comprises coating saidsubstrative layer with at least one so-called semiconductive bufferlayer 140. The materials used correspond to those in the descriptionprovided for buffer layer 140. Said buffer layer may be deposited usinga variety of techniques such as CVD, PVD, sputtering, sintering,electrodeposition, printing, atomic layer deposition, or as a well knowntechnique at atmospheric pressure, chemical bath deposition (CBD). Toform a cadmium sulfide buffer layer, a person skilled in the art mayform at least one buffer layer bath for CBD comprising a Cd salt aqueoussolution of about 0.028 M concentration and an ammonia solution of about14.5 M concentration that are first mixed together with water,preferably high-purity water (18 MΩ·cm), in a volume ratio of about3:7:37, preheated to about 70° C. for about 2 min, and then supplementedwith a thiourea aqueous solution of about 0.374 M concentration. Atleast substrative layer 130 is then immersed within said buffer layerbath that is maintained to a temperature of about 70° C. until thedesired buffer layer thickness is obtained. At least substrative layer130 is then washed with water, preferably high-purity water.

Following at least one step of forming cavities 236, a person skilled inthe art will notice, for example using TF analysis techniques such asX-ray photoelectron spectrometry (XPS) or secondary ion massspectrometry (SIMS), that the surface of the substrative, or absorber,layer comprises more oxidation state +1/+2 elements than the surface ofabsorber layers manufactured according to prior art or, for an analysisusing inductively coupled plasma mass spectrometry (ICP-MS), that theconcentration of the +1/+2 elements in the absorber layer is higher thanin absorber layers manufactured according to prior art.

For example, for devices where the oxidation state +1/+2 element(s)added at the step of forming cavities 236 is cadmium, an ICP-MS analysisof the resulting device after forming cavities is summarized in Table 2.Here, adding at least one alkali metal 235 is provided by evaporatingmaterial comprising Cs and/or Rb fluoride (Cs and/or RbF) to add Csand/or Rb to the substrative layer 130. Three devices are compared: afirst device where no Cs and/or Rb is added, a second, best mode device,subjected to about 20 minutes of evaporation, and a third devicesubjected to about 60 minutes of evaporation. The evaporation rates arethose mentioned previously. Evaporation flux rates and correspondingevaporation durations will be adapted by the person skilled in the art.A person skilled in the art may therefore advantageously use amanufacturing method where adjusting the amount of adding at least onealkali metal 235, or for a more specific example, adding Cs and/or Rb,contributes to regulating the amount of oxidation state +1/+2 elementsthat are added to or absorbed by the absorber layer. Said method ofadding at least one alkali metal 235 may also contribute to adjustingthe duration of the step of forming cavities 236 or steps therein ofaqueous wetting 237, treating of layer surface 238, forming buffer layer240.

TABLE 2 Effect on cadmium concentration from post- deposition adding ofCs and/or RbCs and/or Rb Range Pref, range Cadmium concentration (atomicpercentage) from-to from-to Device with no added Cs and/or Rb  0.02-0.1 0.05-0.4 Dev. with 20 min. added Cs and/or Rb 0.05-2  0.2-1 Dev. with60 min. added Cs and/or Rb 0.1-4 0.5-2

Steps 238 and/or 240 are preferably done in at least one bathingapparatus. However, a person skilled in the art will readily adapt themethod for use with a spraying apparatus. It is also possible to usephysical vapor deposition (PVD) apparatuses and a plurality ofassociated deposition methods, the most common being evaporativedeposition or sputter deposition. A person skilled in the art willreadily manufacture a set of devices covering a range of parameters, forexample a range of durations for at least one of steps 237, 238, 240,244, 246, so as to obtain a photovoltaic device where, for example,photovoltaic efficiency is maximized. Other objectives may be, forexample, to minimize buffer layer thickness, maximize efficiency tobuffer layer thickness ratio, or maximize efficiency to cadmium contentratio.

At optional step 242 at least substrative layer 130 is subject to atleast one short drying step. The short drying step may be done in air,but preferably is done using a blown inert gas, more preferably using atleast one ionization blow off nozzle, even more preferably with theblown inert gas being nitrogen.

At optional step 244 at least substrative layer 130 is subject to atleast one annealing step. For a cadmium sulfide buffer layer, saidannealing step is preferably done at about 180° C., preferably in airfor about 2 min. For a zinc oxi-sulfide buffer layer, said annealingstep is preferably done at about 200° C., preferably in air for about 10min.

At optional step 246, at least substrative layer 130 is subject to atleast one degassing step. This step is ordinarily not needed for deviceswhere the substrate does not absorb humidity, for example glass or metalsubstrates. Said degassing is preferably done in vacuum. The degassingstep is preferred for substrate materials, for example polyimide, thatmay have absorbed humidity at previous manufacturing steps. For example,for a degassing temperature of about 25° C., an effective degassingduration is about 35 hours.

Parameter ranges for steps 237 to 246 are provided in Table 2. To tunethe optional process of forming the buffer layer of step 240, oneskilled in the art will ordinarily develop a test suite over a range ofbuffer coating process durations to manufacture a range of PV devicescomprising a range of buffer layer thicknesses. One will then select thebuffer coating process duration that results in highest PV deviceefficiency.

The following steps describe how to complete the manufacture of aworking PV device benefiting of the invention.

At step 250, forming filler layer comprises coating the device with atleast one filler layer 1360. Said filler layer may for example have acomposition or comprise materials that are similar to those of thebuffer layer 140 or of the front-contact layer 150. Forming the fillerlayer may for example preferably favor filling the concave embossings1300, 1301, so that the process preferably deposits buffer material intothe cavities. The buffer material may be deposited according to thedescription for step 240 of forming buffer layer. The resulting devicetherefore comprises at least one cavity, preferably a plurality ofcavities, that are filled with buffer material. The person skilled inthe art may complement this step with a washing and/or a drying step.The resulting device may subsequently be coated with at least onetransparent conductive front-contact layer 150. Said front-contact layerordinarily comprises a transparent conductive oxide (TCO) layer, forexample made of doped or non-doped variations of materials such asindium oxide, gallium oxide, tin oxide, or zinc oxide that may be coatedusing a variety of techniques such as PVD, CVD, sputtering, spraying,CBD, electrodeposition, or atomic layer deposition.

At optional step 260, forming front-contact grid comprises depositingfront-contact metallized grid traces 160 onto part of transparentconductive front-contact layer 150. Also optionally, said TF PV devicemay be coated with at least one anti-reflective coating such as a thinmaterial layer or an encapsulating film.

The steps may also comprise operations to delineate cell or modulecomponents. Delineation ordinarily comprises cutting grooves intoback-contact layer 120 to provide electrically separate back-contactcomponents. A second delineation step comprises cutting grooves,segments, or holes into at least substrative layer 130. The seconddelineation step may also comprise cutting into at least one offront-contact layer 150, buffer layer 140, or back-contact layer 120. Athird delineation step comprises cutting grooves into at leastfront-contact layer 150. The second and third delineation step may bemade simultaneously. Delineation, also called patterning, is preferablydone using at least one laser.

The invention claimed is:
 1. A method for fabricating patterns on asurface of a device, comprising: exposing the surface of the device toat least one alkali metal, wherein the at least one alkali metalcomprises Cs or Rb, and the device comprises at least one layer; andcontrolling a temperature of the surface to form a plurality ofembossings at the surface, wherein at least a portion of the pluralityof embossings result from a self-assembly process that comprises:forming of a plurality of alkali crystal compounds including compoundsof Cs or Rb and a material of the at least one layer of the device,wherein the alkali crystal compounds are embedded into the surface ofthe at least one layer of the device, and form at least a first line ofthe embossings that are regularly spaced, and that are adjacent andparallel to at least one second line of regularly spaced embossingswithin at least one region of the at least one layer.
 2. The method ofclaim 1, further comprising exposing the surface of the device to anadditional alkali metal comprising K or Na.
 3. The method of claim 1,wherein the exposing the surface of the device to the at least onealkali metal further comprises adding at least one additional elementcomprising S, Se, or Te.
 4. The method of claim 1, wherein the at leastone layer comprises an ABC chalcogenide material, wherein A comprises Cuor Ag, B comprises In, Ga, or Al, and C comprises S, Se, or Te.
 5. Themethod of claim 1, wherein the at least one layer comprisesCu(In,Ga)Se2.
 6. The method of claim 1, wherein controlling thetemperature of the at least one layer comprises controlling thetemperature in a range from about 250° C. to about 380° C.
 7. The methodof claim 1, further comprising coating at least a portion of the atleast one layer with a functional layer.
 8. The method of claim 7,wherein at least one of the embossings is formed at the surface of theat least one layer and a surface of the functional layer.
 9. The methodof claim 1, wherein the at least one layer of the device is disposed ona surface of a substrate that extends between a delivery roll and atake-up roll of a roll-to-roll manufacturing apparatus.
 10. The methodof claim 1, wherein a long axis of at least one embossing in the firstline of the embossings is collinear or approximately collinear with atleast the first line.
 11. The method of claim 1, wherein the at leastthe first line of the embossings is embossed along at least one crystalstriation of at least one region of the surface of the at least onelayer.
 12. The method in claim 1, wherein the at least one region is ona surface of a crystal at the surface of the at least one layer.
 13. Themethod of claim 1, wherein the at least one layer comprises a reactedlayer.
 14. The method of claim 1, wherein the device is a thin-filmoptoelectronic device that is formed on a flexible substrate.
 15. Amethod for fabricating patterns on a surface of a device, comprising:exposing the surface of the device to at least one alkali metal, whereinthe at least one alkali metal comprises Cs or Rb, and the devicecomprises at least one layer; and controlling a temperature of thesurface to form a plurality of embossings at the surface, wherein atleast a portion of the plurality of embossings result from aself-assembly process that comprises: forming of a plurality of alkalicrystal compounds including compounds of Cs or Rb and a material of theat least one layer of the device, wherein the alkali crystal compoundsare embedded into the surface of the at least one layer of the device,and form at least a first line of the embossings that are regularlyspaced, and that are adjacent and parallel to at least one second lineof regularly spaced embossings within at least one region of the atleast one layer; and forming at least one cavity by dissolving at leasta portion of the alkali crystal compound, wherein the method of formingthe at least one cavity comprises: forming at least one buffer layer onthe surface of the at least one layer; exposing the surface of the atleast one layer to one or more oxidation state +1/+2 elements; aqueouswetting of the surface of the at least one layer with a solutioncomprising water; and aqueous wetting of the surface of the at least onelayer with a diluted aqueous ammonia solution with a diluted ammoniamolarity in a range from 0 to 20 M.
 16. The method of claim 15, whereinat least one embossing has a shape of at least one cavity in the atleast one layer.
 17. The method of claim 15, further comprising a stepof filling at least a portion of the at least one cavity by forming afiller layer within the at least one cavity.
 18. The method of claim 17,wherein at least one embossing comprises an electrical point contactbetween the surface of at least one layer and the filler layer, and atleast some material of which is comprised within the at least oneembossing.
 19. A method for fabricating patterns on a surface of adevice, comprising: exposing the surface of the device to at least onealkali metal, wherein the at least one alkali metal comprises Cs or Rb,and the device comprises at least one layer; forming at least oneback-contact layer; providing at least one substrative layer comprisingan absorber layer; and controlling a temperature of the surface to forma plurality of embossings at the surface, wherein at least a portion ofthe plurality of embossings result from a self-assembly process thatcomprises: forming of a plurality of alkali crystal compounds includingcompounds of Cs or Rb and a material of the at least one layer of thedevice, wherein the alkali crystal compounds are embedded into thesurface of the at least one layer of the device, and form at least afirst line of the embossings that are regularly spaced, and that areadjacent and parallel to at least one second line of regularly spacedembossings within at least one region of the at least one layer.
 20. Themethod of claim 19, further comprising drying the absorber layer.